How to calculate Auxiliary Gutter fill 366.22(A),(B)

Categories: Electrical Safety


Sizing a gutter for the proper fit is relatively easy.  I get a lot of calls to make sure that the gutter that they are using is up to code. 

We are allowed by code to fill a gutter to 20%. of its interior cross-sectional area (366.22(A),(B))  This is for both sheet metal and Nonmetallic gutters

Figuring the 20% is easy, for example.

If you have a 6 X 6 gutter, that would have total of 36 sq. in. and 20% of that is 7.2 sq. in. (36  X  20%  =  7.2 sq. in.) of space to use.  Table

5 is used for the area per conductor.  Take a 3/0 THWN conductor it is listed at .2679 sq. in.for each.  if you wanted to know how many could be

used in a 6  X  6 gutter, you can take the 7.2 sq. in. and   divide it by .2679 to get the maximum number that would be allowed to be installed in

that gutter.  7.2  /  .2679  =  26 conductors. 

You would also have to check the wire bending space minimum from Table 312.6(A). which would require a minimum of 4’” for a 3/0 conductor.


Till next time, work safe always check the circuit with a tester.

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